Conventionally, various photosensitive compositions have been used for formation of a visible image or as a lithographic printing plate material. In particular, recent development of lasers for lithographic printing has been remarkable, and in particular solid-state lasers/semiconductor lasers with an emission range in the near-infrared to infrared that have high output and small size are readily available. These lasers are very useful as an exposure light source when making a plate directly from digital data of a computer, etc.
A positive-working lithographic printing plate precursor for infrared laser comprises as essential components an alkali-soluble binder resin and an infrared absorbing agent, etc., which absorbs light and generates heat. This infrared absorbing agent, etc. functions in an unexposed area (image area) as a development inhibitor, which substantially suppresses solubility of the binder resin in a developer by virtue of interaction with the binder resin; in an exposed area (non-image area) the interaction between the infrared absorbing agent, etc. and the binder resin weakens due to the generated heat and dissolution in an alkali developer progresses, thus forming a lithographic printing plate. However, such a positive-working lithographic printing plate material for infrared laser does not have sufficient processability (development latitude) when a developer is exhausted and the activity drops off.
In order to solve such a problem with development latitude, use of a recording layer comprising a material that can be more easily developed in a non-image area, that is, has better solubility in aqueous alkali solution could be considered, but such a recording layer has the problems that it becomes chemically weak in an image area also and has poor durability in normal printing and, furthermore, has poor chemical resistance such that it is easily damaged by a developer or an ink washing solvent, a plate cleaner, etc. used during printing.
As a solution for these problems, a method in which a recording layer is formed as multiple layers is known (JP-A-11-218914 (JP-A denotes a Japanese unexamined patent application publication)).
Furthermore, as a solution for these problems, a method in which a recording layer is formed as multiple layers and an upper layer and/or a lower layer thereof employs a polyurethane resin having good solubility in aqueous alkali solution and high durability has been used (JP-A-2003-177533 and JP-A-2007-17913).
On the other hand, for the purpose of improving solvent resistance and durability, a lithographic printing plate employing a polyurethane resin having an acidic hydrogen atom-containing macromonomer unit in a side chain has been disclosed (JP-A-10-312058).